Process for the metallization and/or brazing with a silicon alloy of parts made of an oxide ceramic unable to be wetted by the said alloy

ABSTRACT

The invention relates to a process for the metallization with a silicon alloy melting at a temperature T1 of certain zones of the surface of a part made of an oxide ceramic unable to be wetted by the said alloy, the said process comprising, in succession, a step of depositing carbon on the said zones of the said part that are to be metallized, a step of depositing the silicon alloy in solid form on at least one portion of the said part, so that the said alloy has at least one point of contact with the said zones to be metallized, followed by a heating step at a temperature greater than or equal to T1, the said alloy gathering in the molten state on the said zones to be metallized. 
     This process applies also to the brazing of parts, at least one of which is a part made of an oxide ceramic unable to be wetted by the said alloy. 
     Application of the said processes to the fields of electronics, electrical engineering, thermal engineering and chemical engineering.

DESCRIPTION

1. Technical Field

The present invention relates to a process for metallization of partsmade of an oxide ceramic with a metal alloy and to a process for joiningparts by brazing with the said alloy, at least one of the parts beingmade of an oxide ceramic.

This invention is especially applicable in the field of electronics, inparticular for oxide ceramics used as electrical insulators.

2. Prior Art

The technical field of the invention may be defined as that of themetallization and the brazing of ceramic parts.

The term “metallization” should be understood to mean the action ofcoating the surface of a part with a thin metal or metallic alloy layer.

The term “brazing” is understood to mean the joining of two or moreparts by brazing, that is to say by a hard solder joint obtained byinterposing a fusible metallic alloy or metal between the said parts tobe joined.

In general, the metallization or brazing of ceramic parts poses a numberof problems insofar as the ceramics commonly used have the particularfeature of being unable to be wetted by most metallization or brazingcompositions,

such as compositions based on silver, copper, gold or alloys thereof,whereas these same compositions readily wet metallic materials. Thisproblem may be surmounted by carrying out a surface treatment of theceramic before the metallization or brazing.

A first type of surface treatment may consist of metallizing the saidceramic, that is to say a thin layer of metal is deposited whichconstitutes a tie layer for the metallization or brazing composition.

The process most widely used for metallizing or brazing ceramics,especially alumina-based ceramics, is the process called“moly-manganese” described by K. White and D. Kramer in MaterialsScience and Engineering, 75 (1985) 207–213, “Microstructures and SealStrength Relation in the Molybdenum-Manganese Glass Metallization ofAlumina Ceramics” [1]. In this process, a suspension containing a blendof manganese and molybdenum powders is, in a first step, applied to thesurface of the ceramic part and, in a second step, undergoes combustionin a wet and reducing atmosphere of hydrogen and ammonia. Such anatmosphere is needed to maintain the molybdenum in the metal state andallow the manganese metal to oxidize, thanks to the presence of acertain water vapour content.

However, although this process works with ceramics having an aluminacontent of 94–96%, it does not work with ceramics having an aluminacontent of 99.5%. To metallize or braze such surfaces, it is suggestedin that document to add a manganese-based glass (MnO—SiO₂—Al₂O₃) to themolybdenum powder, instead of pure manganese metal. During the heattreatment, the manganese-based glass penetrates into the grainboundaries of the ceramic and forms a glassy matrix in which themolybdenum particles are trapped, thus promoting the metallization. Theglass deposit, containing metal inclusions, is then metallized byelectrolytically depositing a nickel layer and the subsequent brazing isthen carried out using a nickel-based brazing composition.

This process has the drawback of being expensive and complex, especiallybecause it involves many successive treatments of the ceramic surface.

Another type of surface treatment may consist in depositing anon-metallic layer on the ceramic part to be metallized or brazed, thedeposition of this layer taking place prior to metallization or brazing,the said layer constituting a tie layer for the metallization or brazingcomposition.

Thus, U.S. Pat. No. 4,636,434 [2] discloses a process for joining acomposite, conformed from at least one element made of a ceramic withanother element made of a ceramic or a metal, the said processconsisting in joining the said elements together by means of a metaljoint formed between the surfaces of the said elements to be joinedtogether. In a first step, a carbon film is formed on the ceramicsurface or surfaces to be joined. This carbon film is obtained byapplying an organic substance (such as organic solvents, resins) on thesurface or surfaces to be joined, followed by heating in a non-oxidizingatmosphere. In a second step, a metal film is formed on the carbon film,the said metal film being deposited using various techniques, such asmetallization by sputtering. Finally, the surfaces thus covered arejoined together with the interposition of a braze between the saidsurfaces, followed by suitable heating. The braze described is onechosen from silver-based, copper-based, nickel-based, brass-based andiron-based materials.

However, this process has the following drawbacks:

-   -   it requires the application of the brazing composition to the        entire carbon-coated surface, so as to guarantee the formation        of the brazed joint;    -   it employs brazing compositions based on low-melting-point        metals, which means that the resulting brazed joints are not        very refractory.

Patent FR 2 328 678 [3] discloses a process for the metallization ofceramic parts with pure silicon.

This process comprises, in succession, the following steps:

-   -   a step of coating a surface to be metallized of a ceramic        substrate with carbon; and    -   a step of bringing the carbon-coated surface into contact with        molten pure silicon in order to form a silicon layer on the        carbon-coated surface.

The contacting step is carried out by dipping the carbon-coatedsubstrate into a bath of molten pure silicon, the substrate then beingremoved from the bath at a rate allowing the silicon to crystallize onthe substrate at the points covered with carbon.

This process has the following drawbacks:

-   -   because of the very high wetting angle (which may possibly be        around 50–60°) made between the pure silicon and a plane ceramic        surface, to overcome this drawback it is necessary to employ a        very complex dipping-extracting technique in order to obtain a        coating on the carbon areas, as was explained above; and    -   to obtain complete metallization of the carbon-coated areas, it        is necessary to cover the entire surface of the carbon-coated        areas with pure silicon, this being accomplished by immersing        the entire ceramic part to be metallized into a bath of molten        silicon.

Other processes may consist in using what are referred to as reactivecompositions, in which elements highly reactive with respect to oxideceramics are incorporated, the said elements being chosen from Ti, Zr,Hf, Nb, Al and Cr.

For example, mention may be made of the “titanium hydride” process whichconsists, in a first step, in reacting titanium hydride with the oxideceramic, in order to activate the surface of the said ceramic, and then,in a second step, in depositing the metallization layer on the activatedsurface or depositing the braze, if it is desired to join the partstogether.

However, the use of titanium hydride is extremely tricky, in so far astitanium hydride is a very unstable compound, and requires aspecial-purpose heater, making the said use not very practicable formany industrial applications.

Among metallization or brazing processes using a reactive composition,mention may also be made of processes using “ready-to-use” reactivealloys that require no prior surface treatment of the ceramics to betreated.

Thus, the document European Patent EP 0 135 603 [4] mentions the use ofbraze alloys containing 0.25% to 4% by weight of a reactive elementchosen from titanium, vanadium, zirconium and inter alia from 20 to 80%by weight of silver and other elements. The use of such an alloy doesnot require the prior treatment of the surface to be metallized orbrazed. However, such alloys melt at temperatures ranging from 600 to950° C., which sets them aside for applications at high temperatures,for example greater than 1000° C. In addition, these alloys have a verypoor oxidation resistance above 500° C.

Patent FR 2 787 737 [5] presents another type of reactive brazingcomposition, in this case refractory, for the brazing of alumina,comprising a matrix made of palladium or nickel or a nickel-palladiumalloy, with the addition of titanium and aluminium in all three cases.

Thus, the processes for metallizing or brazing ceramic parts of theprior art all have one or more of the following drawbacks;

-   -   they require the use of very complex techniques, such as        dipping-extraction (as is the case for document FR 2 328 678)        with control of the rate of extraction in order to obtain        correct metallization;    -   when they employ a tie layer (for example a metal layer or a        carbon layer), the processes of the prior art require the said        tie layer to be completely covered by the metallization or        brazing composition in order to deposit metal over the entire        surface of this layer;    -   they result, because of the metallization or brazing        compositions used, in the formation of metallization layers or        brazed joints that are barely refractory; and    -   they can be applied only to parts of simple design, the areas of        which that are to be metallized or joined not having machined        parts, such as channels for example.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide a processfor metallizing and a process for brazing parts made of an oxide ceramicthat do not have the drawbacks of the prior art, both these processeshaving the particular feature of making it possible, using the samecomposition, to selectively metallize or braze certain zones of an oxideceramic surface not wettable by the said composition.

The Applicant has thus surprisingly discovered that by using a alloy ofsilicon and at least one other metal, it was possible to metallize orbraze only specific zones of ceramic parts, the said zones being coveredbeforehand with a tie layer, this “selective” metallization or brazingbeing achieved by the simple fact of depositing the said alloy on aportion of the ceramic part to be metallized or brazed.

Thus, according to a first subject, the invention relates to a processfor the metallization with a silicon alloy melting at a temperature T1of certain zones of the surface of a part made of an oxide ceramicunable to be wetted by the said alloy, the said process comprising, insuccession, the following steps:

-   -   deposition of carbon on the said zones to be metallized of the        said part;    -   deposition of the silicon alloy in solid form on at least one        portion of the said part, so that the said alloy has at least        one point of contact with the said zones to be metallized; and    -   heating to a temperature greater than or equal to T1, the said        alloy thus gathering in the molten state on the said zones to be        metallized.

Thus, by using a alloy of silicon and at least one other element it isunnecessary to distribute the alloy over the entire surface of the zonesprecoated with carbon that are to be metallized (the alloy having onlyto have at least one point of contact with the said zones to bemetallized) insofar as this alloy is capable, in the molten state, thatis to say after being heated to a temperature of greater than or equalto T1 (T1 corresponding to the melting point of this alloy) ofspontaneously gathering on the carbon-covered zones. This alloymetallizes only the carbon-covered zones, leaving the zones not coveredby the said carbon intact, thus forming a continuous metal deposit ofuniform thickness.

The ability of these alloys to move in the molten state from one pointon the oxide ceramic part to another, in order to wet only thecarbon-covered zones, is due to the very low wetting angle (generallyless than 30°) that these alloys in the molten state, just prior tosolidification, make with an oxide ceramic surface. This capability doesnot occur with pure silicon, which in the molten state makes a highwetting angle with an oxide ceramic surface (this angle possibly beingaround 50–60°)—metallization with pure silicon thus requires verycomplex deposition techniques, as explained in document FR 2 328 678.

Advantageously, this process, by using silicon alloys, makes it possibleto obtain, after the molten alloy has cooled, metallization layersresistant to very high temperatures, particularly temperatures above1200° C.

Finally, this metallization process is very simple to implement.

The step of depositing the silicon alloy in solid form may, for example,be carried out on the entire surface of the part. The said alloy, afterheating, will then spontaneously withdraw from the carbon-free zone toconcentrate in the zones to be metallized.

This deposition step may also, according to the invention, be carriedout directly on only one portion of the zones to be metallized, thecondition being that this alloy should have at least one point ofcontact with the said zones to be metallized, that is to say thecarbon-covered zones.

According to a second subject, the invention also relates to a processfor joining two parts by brazing them over certain zones of theirsurfaces with a silicon alloy melting at a temperature T1, at least oneof the parts being made of an oxide ceramic unable to be wetted by thesaid alloy, the said process comprising, in succession, the followingsteps:

-   -   contacting of the surfaces of the said parts with the silicon        alloy in solid form, the said zones to be joined together of the        surface of the unwettable part(s) made of an oxide ceramic being        covered beforehand with carbon, the said alloy having at least        one point of contact with the said carbon-covered zones; and    -   heating of the assembly formed by the said parts and the        composition to a temperature greater than or equal to T1, the        said alloy thus gathering, in the molten state, on the zones to        be joined.

It should be pointed out that, for the same reasons as mentioned in thecase of the first subject, it is unnecessary to deposit the alloy insolid form over all of the zones to be joined. The alloy may besandwiched between the parts to be brazed, but it may also be positionednear the zones to be joined, having only one point of contact with thesaid zones to be joined. During heating, the silicon alloy, because ofits low wetting angle, has the ability to move from one point to anotheruntil wetting only the carbon-covered zones and constituting, aftercooling, a brazed joint.

It should be pointed out that, according to the invention, the parts tobe brazed may be only parts made of non-wettable oxide ceramics, inwhich case the steps of covering with carbon will have to be carried outon those zones of each part that are to be joined. It should be notedthat the parts able to be joined with at least one oxide ceramic partaccording to the invention may also be parts made of silicon carbide,silicon nitride or aluminium nitride.

This brazing joining process according to the invention guaranteescontrolled brazing with perfectly controlled spreading of the alloy overthe zones to be joined, in particular avoiding the formation of brazefillets, dewetting bubbles or leaks of braze onto the zones not to bejoined.

In addition, this process, by using a silicon alloy, makes it possibleto obtain brazed joints that are resistant to very high temperatures,for example temperatures above 1200° C., which is not the case with theprocesses of the prior art that employ low-melting-point alloys, as isthe case for silver-based or copper-based alloys.

According to the invention, whether as regards the first or the secondsubject, the oxide ceramic may be chosen from alumina-based ceramics,silica-based ceramics and aluminosilicate ceramics, such as mullite andcordierite.

According to the invention, the carbon to be deposited may be applied byany technique for obtaining a carbon deposit adhering to an oxideceramic surface.

Thus, according to a first variant of the invention, the carbon may bedeposited in the form of graphite powder that may or may not be blendedwith an organic binder.

According to a second variant of the invention, the carbon may bedeposited by various deposition techniques such as CVD (Chemical VapourDeposition) and PVD (Physical Vapour Deposition). CVD and PVD techniqueshave the advantage of being techniques that can be completely controlledfrom an industrial standpoint and for which it is possible, inparticular, to use masks for delimiting the zones to be metallized orbrazed from the zones to be left intact.

Finally, according to a third variant of the invention, the carbon maybe deposited by rubbing with a graphite ore.

Preferably, according to the invention an amount of carbon ranging from0.1 mg/cm² to 1 mg/cm² is deposited.

The fact of using carbon in the process of the invention is particularlybeneficial insofar as this material is very abundant and inexpensive.

Once the areas to be metallized or brazed have been coated with carbon,it is necessary according to the invention to deposit the silicon alloyable to wet the carbon deposited beforehand.

To obtain a continuous metal coating or brazed joint of uniformthickness, the silicon alloy used within the context of the inventionmust be able to wet the carbon deposited on the oxide ceramic surfacewhile still having the ability to move from one point of an oxideceramic part to another, so as to selectively wet the carbon-coveredzones. The choice of such an alloy according to the abovementionedcriteria lies within the competence of a person skilled in the art.Advantageously, the silicon alloy used in the process of the inventionhas a silicon content of greater than 56 at % (at % means % in atomicweight).

According to the invention, the silicon alloy may advantageouslyfurthermore contain at least one metal element chosen from Co, Zr, Ti,Rh, V, Ce, Cr, Re, Ru, Y, Hf, Ir and Ge.

In particular, the silicon alloy may be chosen from the alloys havingthe following compositions:

-   -   Co-containing silicon alloys with a silicon content ranging from        58 to 97 at %;    -   Zr-containing silicon alloys with a silicon content ranging from        87 to 97 at %;    -   Ti-containing silicon alloys with a silicon content ranging from        76 to 97 at %;    -   Rh-containing silicon alloys with a silicon content ranging from        58 to 97 at %;    -   V-containing silicon alloys with a silicon content ranging from        95 to 97 at %;    -   Ce-containing silicon alloys with a silicon content ranging from        81 to 97 at %;    -   Cr-containing silicon alloys with a silicon content ranging from        75 to 97 at %;    -   Re-containing silicon alloys with a silicon content ranging from        88 to 97 at %;    -   Ru-containing silicon alloys with a silicon content ranging from        81 to 97 at %;    -   Y-containing silicon alloys with a silicon content ranging from        75 to 97 at %;    -   Hf-containing silicon alloys with a silicon content ranging from        84 to 97 at %;    -   Ir-containing silicon alloys with a silicon content ranging from        60 to 97 at %;    -   Ge-containing silicon alloys with a silicon content ranging from        60 to 97 at %.

To give an example, according to the invention, when the metallizationor the brazing applies to an alumina part, it is possible to use asilicon alloy containing 90 at % Si and 10 at % Zr.

This type of silicon-rich alloy has the advantage of making it possibleto obtain particularly refractory metallization zones or brazed jointswhich in particular exhibit satisfactory oxidation resistance andmechanical properties at temperatures above 1000° C.

This type of alloy also has the advantage of being “non-reactive” withrespect to oxide ceramics, especially those of the aluminosilicate type,that is to say they do not attack the surface of the said ceramics, thisallowing metallization or brazing and demetallization or “debrazing” ofthe said ceramics, it being possible for the demetallization or“debrazing” to be accomplished by simply chemically etching with amixture of acids. Thus, such alloys make it easier, for example, torepair the metallized or brazed zones. For example, this type of alloycan be dissolved by chemical etching with hydrofluoric acid, and theareas thus demetallized or “debrazed” may be rebrazed or remetallized.

According to the invention, the silicon alloy in solid form may be inthe form of a powder blended with an organic binder, especially when itis brittle, as is the case when the constituent ingot of the siliconalloy can be crushed into a powder. The silicon alloy in solid form mayalso be in the form of a foil, especially when it is ductile.

According to the invention, during the final step of the metallizationprocess and the brazing process, when the heating is carried out this ispreferably done in a furnace under vacuum or in an inert gas atmosphere.

For example, the inert gas is chosen from argon or nitrogen.

The fact of working in an inert gas atmosphere or under vacuum isparticularly advantageous according to the invention insofar as thisavoids the possible formation of any oxide layer during heating, whichwould contribute to downgrading the wetting of the metallizationcomposition on the carbon-covered zones of the oxide ceramic.

The metallization process or the brazing joining process are, accordingto the invention, particularly beneficial in so far as they can becarried out in many sectors.

Thus, in the field of just metallization, the main application of theprocess according to the invention is in electronics and electricalengineering for producing integrated circuit substrates that withstandextreme temperature and oxidation conditions.

In the braze joining field, the applications of the process according tothe invention are numerous. Mention may be made, among others, ofthermal engineering, especially for heat exchangers and condensers,chemical engineering, especially for chemical reactors, and mechanicalengineering, especially for friction parts and machine/cutting tools.

The invention will now be described with reference to the followingexamples, these being given by way of illustration but implying nolimitation.

EXAMPLE 1

A silicon-based alloy having a silicon content of 90 at % and azirconium content of 10 at % was prepared from powders. The alloy wasthen blended with an organic binder so as to obtain a paste.

In parallel, the surface of an alumina disc 25 mm in diameter wasprepared by cleaning it and then drying it. Carbon was deposited on thisdisc using a graphite ore, the deposit being in the form of a cross withbranches 1 cm in length and 1 mm in width. The centre of this cross wascovered with 300 mg of the alloy/binder blend, without covering thebranches. The assembly was put into a vacuum furnace and heated to atemperature of 1420° C. for 5 minutes, so as to melt the blend. Afterthis heat treatment, it was found that the blend had only covered thecross without covering the non-graphitized zone.

EXAMPLE 2

An alloy based on silicon and zirconium was prepared, and then mixedwith a binder in accordance with Example 1.

The surface of an alumina disc 25 mm in diameter was prepared bycleaning it followed by drying. Next, carbon was deposited by rubbingthe surface with a graphite ore over one half of the alumina disc. 400mg of the alloy/binder blend were then placed at the centre of the disc,so as to cover both the graphitized zone and the non-graphitized zone.The assembly was heat treated in accordance with Example 1. It was foundthat, after this treatment, the alloy had departed from thenon-graphitized zone and was found only in the graphitized zone.

EXAMPLE 3

A silicon alloy having a silicon content of 77.5 at % and a cobaltcontent of 22.5 at % was prepared. The alloy was then blended with anorganic binder, so as to obtain a paste. The surface of an aluminawasher was treated and covered with carbon in a cross-shaped pattern asin Example 1. The alloy/binder blend was deposited at the centre of thecross, that is to say at the intersection of the branches, withoutcoating the branches therewith. The assembly was placed in a vacuumfurnace and heated to a temperature of 1380° C. for one minute. Afterthis heat treatment, the alloy had completely covered the cross, whileleaving the zones not covered with carbon intact.

EXAMPLE 4

A silicon alloy having a silicon content of 66.67 at % and a cobaltcontent of 33.33 at % was prepared. The alloy was then blended with anorganic binder so as to obtain a paste. The surface of an alumina discwas treated and completely covered with carbon as in Example 1. Thealloy/binder blend was deposited at the centre of the disc. The assemblywas placed in a vacuum furnace and heated to a temperature of 1380° C.for one minute. Upon removal from the furnace, the alloy was found tohave spread perfectly and covered the entire surface of the discinitially coated with carbon. The measured wetting angle of this meltedalloy with the surface of the oxide ceramic was less than 30°, whichexplains the perfect spreading of this alloy over the carbon-coveredzone.

COMPARATIVE EXAMPLE 1

The surface of an alumina disc was covered with carbon in the form of across as per Example 1. Pure silicon (more precisely, a blend of puresilicon powder with an organic binder) was deposited at the centre ofthe cross, without covering the branches. The assembly was placed in avacuum furnace and heated to a temperature of 1420° C. for one minute.After this heat treatment, it was found that the silicon had spread overthe cross in a discontinuous fashion and that the deposit had a variablethickness. A considerable mass of silicon was still present at thecentre of the cross. The measured wetting angle of the pure silicon wasaround 50–60°, too high a value to allow perfect spreading of thesilicon.

1. A process for the metallization with a silicon alloy melting at atemperature T1 of certain zones of the surface of a part made of anoxide ceramic unable to be wetted by said alloy, said oxide ceramicbeing an alumina-based ceramic and said alloy being a mixture of siliconand at least one metal element chosen from Co, Zr, Ti, Rh, V, Ce, Cr,Re, Ru, Y, Hf, Ir and Ge, said process comprising, in succession, thefollowing steps: deposition of carbon on the said zones to be metallizedof said part; deposition of the silicon alloy in solid form on at leastone portion of said part, so that said alloy has at least one point ofcontact with said zones to be metallized; and heating to a temperaturegreater than or equal to Ti, said alloy thus gathering in the moltenstate on the said zones to be metallized.
 2. A process for joining twoparts by brazing them over certain zones of their surfaces with asilicon alloy melting at a temperature T1 , at least one of the partsbeing made of an oxide ceramic unable to be wetted by said alloy, saidoxide ceramic being an alumina-based ceramic and said alloy being amixture of silicon and at least one metal element chosen from Co, Zr,Ti, Rh, V, Ce, Cr, Re, Ru, Y, Hf, Ir and Ge, said process comprising, insuccession, the following steps: contacting of the surfaces of saidparts with the silicon alloy in solid form, said zones to be joinedtogether ef at the surface of the unwettable part(s) made of an oxideceramic being covered beforehand with carbon, said alloy having at leastone point of contact with said carbon-covered zones; and heating of theassembly formed by said parts and the said alloy to a temperaturegreater than or equal to T1 , said alloy thus gathering, in the moltenstate, on the areas zones to be joined.
 3. The process as claimed inclaim 1 or 2, in which the carbon is deposited in the form of graphitepowder that may or may not be blended with an organic binder.
 4. Theprocess as claimed in claim 1 or 2, in which the carbon is deposited byphysical vapour deposition or chemical vapour deposition.
 5. The processas claimed in claim 1 or 2, in which the carbon is deposited by rubbingwith a graphite ore.
 6. The process as claimed in claim 1 or 2, in whichan amount of carbon ranging from 0.1 mg/cm² to 1 mg/cm² is deposited. 7.The process as claimed in claim 1 or 2, in which the silicon alloy has asilicon content of greater than 56 at %.
 8. The process as claimed inclaim 1, in which the silicon alloy is chosen from the alloys having thefollowing compositions: Co-containing silicon alloys with a siliconcontent ranging from 58 to 97 at %; Zr-containing silicon alloys with asilicon content ranging from 87 to 97 at %; Ti-containing silicon alloyswith a silicon content ranging from 76 to 97 at %; Rh-containing siliconalloys with a silicon content ranging from 58 to 97 at %; V-containingsilicon alloys with a silicon content ranging from 95 to 97 at %;Ce-containing silicon alloys with a silicon content ranging from 81 to97 at %; Cr-containing silicon alloys with a silicon content rangingfrom 75 to 97 at %; Re-containing silicon alloys with a silicon contentranging from 88 to 97 at %; Ru-containing silicon alloys with a siliconcontent ranging from 81 to 97 at %; Y-containing silicon alloys with asilicon content ranging from 75 to 97 at %; Hf-containing silicon alloyswith a silicon content ranging from 84 to 97 at %; fr-containing siliconalloys with a silicon content ranging from 60 to 97 at %; Ge-containingsilicon alloys with a silicon content ranging from 60 to 97 at %.
 9. Theprocess as claimed in claim 1 or 2, in which the silicon alloy in solidform is in the form of a powder blended with an organic binder.
 10. Theprocess as claimed in claim 1 or 2, in which the silicon alloy in solidform is in the form of a foil.
 11. The process as claimed in claim 1 or2, in which the heating is carried out in a furnace under vacuum or inan inert gas atmosphere.